The free electron laser is a high power coherent radiation source. In a free electron laser a beam of relativistic electrons is caused to pass through a static periodic magnetic field with resultant amplification of a superimposed coherent optical input. The electrons in the beam are accelerated and decelerated and the laser action is thought to result from stimulated Compton backscattering of the virtual photons in the periodic magnetic field, or stimulated magnetic bremsstrahlung. Unlike atomic lasers, which provide a coherent light output at only a single frequency related to the energy levels of electrons in the atom, free electron lasers are continuously tunable within a range by varying the energy of the beam of electrons and/or by changing the parameters of the periodic magnetic field. For more information regarding the operation and structure of such a free electron laser, reference may be made to U.S. Pat. No. 3,822,410.
A gyrotron is a form of microwave generator based upon the cyclotron maser interaction between an electromagnetic wave and a beam of relativistic electrons in which the individual electrons move along helical paths in the presence of the applied magnetic field. Cyclotron resonance coupling offers the advantage that both the electron beam and the microwave structures can have dimensions which are large compared to the output wavelength. One of the primary uses for a high power gyrotron is considered to be for fusion ignition. For further information regarding the operation of a gyrotron, reference may be made to U.S. Pat. No. 3,398,376.
One proposed free electron laser includes a corkscrew field magnet adjacent the input end of the drift tube for causing relativistic electrons to spiral. This laser also includes an azimuthal magnet system causing the spiraling electrons to undergo accelerations. More specifically, this magnet system causes a scalloped movement of the spiraling electrons. For further information concerning the structure and operation of such a laser, known as a gyro free electron laser, reference may be made to U.S. Pat. No. 4,679,197.
Considered possibly the most efficient type of free electron laser is the cyclotron autoresonance maser (CARM). In the CARM, the wave phase velocity is at least substantially as great as the velocity of light in a vacuum. For further information on the CARM, reference may be made to "Relativistic gyrotrons and cyclotron autoresonance masers", V. L. Bratman et al., Int. J. Electronics. 51, 541 (1981); and "Induced resonance electron cyclotron quasi-optical maser in an open resonator", P. Sprangle et al., Appl. Phys. Lett., 49, 1154 (1986). In the CARM discussed in the Bratman et al. paper, the interaction between the electron beam and the radiation beam occurs in a waveguide. The electron beam proceeds along the axis of the waveguide and the radiation reflects off the walls of the waveguide so that the beams periodically cross as the beams travel down the waveguide. With the radiation beam crossing the electron beam at an angle of about 25 degrees, the phase velocity is about 1.1 times the speed of light. The disadvantages attendant the use of a waveguide are that the cross-sectional dimensions of the waveguide must be small, additional modes of electromagnetic wave oscillation may be excited, and the multiple radiation beam reflection occasions heating of the waveguide wall.
A high power radiation source for the frequency range of 140 GHz to 560 GHz has several potential applications. Electron cyclotron resonance heating (ECRH) of high-field tokamaks requires microwave sources in this frequency range that have not been available. For a 10 tesla tokamak, fundamental ECRH heating at 280 GHz is possible if the plasma density is sufficiently low. Second harmonic ECRH heating at 560 GHz may be preferable to avoid constraints on the plasma density. This type of radiation source could also be used for sintering ceramics and for other materials applications requiring efficient absorption of the microwave energy. The absorption length at 280 GHz for even the lowest loss ceramics is less than 10 cm at room temperature. The absorption length decreases with frequency and temperature. Thus sources in the range of 140 GHz to 560 GHz can be very effective in heating thin ceramics. Radar is another potential application. A radar operating above 200 GHz would have much greater resolution than conventional lower frequency radar.
For this frequency range, the CARM has several theoretical advantages relative to its major competitors, the conventional free electron laser and the gyrotron. The conventional free electron laser requires a much higher voltage to produce radiation of the same frequency as a CARM because of the limitations on the minimum size and the maximum field strength of a practical wiggler magnet. The gyrotron operates at a low voltage but requires a higher magnetic field to produce radiation of the same frequency as a CARM. The gyrotron frequency is downshifted as a function of the relativistic factor (the relativistic angular frequency of electron motion about the magnetic field [.omega.] is proportional to the reciprocal of the relativistic enhancement of the electron mass [.gamma.]) whereas in a CARM, the frequency is upshifted (roughly, .omega..about..gamma.). Taking a maximum magnetic field strength of 10 tesla in a gyrotron device, the maximum fundamental frequency for a non-relativistic gyrotron is 280 GHz.